As one modulation scheme for optical communication systems, there has been employed a direct modulation scheme for modulating a laser diode with a driving electric current to obtain light intensity signals proportional to electrical signals. This direct modulation scheme has an advantage of significantly-simple device structure. However, in ultra-high-speed/broad-band optical systems with transmission speeds larger than several Gbits per second, there have been induced wavelength fluctuation (chirping) phenomena that light wavelengths vary during direct modulations, which has imposed limitation on transmission capacities. Therefore, the direct modulation scheme has been utilized for optical communication systems with relatively lower speeds.
On the other hand, as optical modulation schemes for ultra-high speed transmissions, in order to suppress the chirping associated with the modulation, there have been employed outside modulation schemes adapted to continuously emit light from a semiconductor laser and to turn on and off the light with an outside modulator. Most ordinary modulators as such outside modulators are Mach-Zehnder modulators.
MZ modulators have the advantage of less chirping, but have induced the problem of inter-code interference in ON/OFF levels of light outputs, due to temperature changes, temporal changes and the like. There has been a need for overcoming this problem for stably controlling operating points in such Mach-Zehnder modulators. For example, Japanese Patent Laying-Open No. 2014-10189 (PTL 1), Japanese Patent Laying-Open No. 2012-257164 (PTL 2) and the like disclose bias stabilizing control through modulation schemes using NRZ (Non Return to Zero) codes and the like. These literatures suggest compensation techniques for detecting the amount of variation of an operating point and the direction of variation thereof through a low-frequency signal superposed on a driving voltage and for controlling the bias voltage through feedback to normally maintain the operating point. According to these controls, an optimum bias point is a substantially-intermediate point between a voltage (PEAK point) that turns on the light output and a voltage (NULL point) that turns off the light output.
Under the aforementioned circumstance, there has been also a need for bias stabilizing control in modulation schemes where an optimum driving voltage amplitude is 2Vπ, such as BPSK (Binary Phase Shift Keying) or CSRZ (Carrier-Suppressed Return to Zero), and there have been suggested compensation techniques for detecting the amount of variation of an operating point and the direction of variation thereof through a dither (low-frequency signal) superposed on a bias voltage and for controlling the bias voltage through feedback to normally maintain the operating points.
In cases of ideal MZ modulators, in BPSK or CSRZ, the optimum bias point is at the voltage that turns off the light output (hereinafter, referred to as a NULL point) and, therefore, bias control is performed such that the bias point comes to be at the NULL point.